Referring to FIGS. 1a to 1c, there is shown a conventional method of retrieving a core sample 10 created by a core drilling system 12. In this example, the drilling system 12 is drilling downwards in a substantially vertical orientation, although it will be appreciated that drilling may be performed in any appropriate orientation, for example at any angle between +35° to −90° with respect to a horizontal plane.
The core sample 10 is created when an annular drill bit 14 drills through ground 16. The drill bit 14 is coupled to a lower end of an outer tube assembly 18, which in turn is arranged at a lower end of a drill string 20 which extends to a collar point of the drilling system 12. In this example, the ground 16 is at a bottom of a borehole 22 that has been drilled by the core drilling system 12.
An inner tube assembly 24 is typically used to retrieve the core sample 10. The inner tube assembly 24 is deployed down (FIG. 1a) the drill string 20. The inner tube assembly 24 engages with the outer tube assembly 18 (FIG. 1b) and the core sample 10 is then created by drilling through the ground 16 (FIG. 1c). The inner tube assembly 24 retains the core sample 10 when the core sample 10 is created and, after the core sample 10 is broken off from the ground 16, the inner tube assembly 24 is retrieved from down the drill string 20.
Typically, the inner tube assembly 24 comprises a core tube assembly 26 that is arranged to retain the core sample 10, and a head assembly 28 that is arranged to facilitate deploying the inner tube assembly 24 down the drill string 20, and that is further arranged to facilitate retrieving the inner tube assembly 24 from down the drill string 20. To achieve this, the head assembly 28 is arranged such that it can be coupled to the core tube assembly 26 and sent down the drill string 20, for example by pumping water down the drill string 20 towards the inner tube assembly 24, or by dropping the inner tube assembly 24 down the drill string 20 if the drill string is in a substantially vertical orientation. Once the inner tube assembly 24 has been deployed down the drill string 20, the head assembly 28 engages with the outer tube assembly 18 and drilling can then commence. After the drilling has created the core sample 10 and the core sample 10 is retained in the core tube assembly 26, an overshot 30, which is coupled via a wireline 32 to a winch (not shown) located at the collar point, is deployed along the drill string 20 to engage with the head assembly 28. The overshot 30 is then winched to the collar point, bringing the inner tube assembly 24 and the core sample 10 to the collar point for retrieval.
The retrieval of core samples is a limiting factor in the time taken to perform core drilling, and the time taken to retrieve the core samples increases as the drilling depth increases.